I haven't worked with reactors, but that might be a way to accomplish it, if a reactor can be triggered by setting a particular Layer current.  It could presumably set the current-entity linetype scale to what you want, so that new things drawn on that Layer would be at that scale.  But whether the linetype scale could be set back [presumably to 1] when you change to any different current Layer, I'm not sure -- someone with more experience with reactors could probably say.

 

Routines could also be made [without reactors] that would do that linetype-scale setting change when called, allow you to draw some specific thing, and set the linetype scale back when finished.  For instance, you could have a command for drawing a Line on that Layer, and one for drawing a Circle on that Layer.  One could probably also contain entity-type options within such a command, to set the Layer and linetype scale for whatever among those options you choose, and set both back when you're done.  Would something like that work?